1. Field of the Invention
This invention relates generally to a printed circuit, and more particularly is directed to a printed circuit suitable for use as an electric wave filter.
2. Description of the Prior Art
Radio receivers and television receivers customarily employ a number of resonant circuits which are used as electric wave filters to select desired frequency signal. Such filters commonly consist of primary and secondary coils and a capacitor, and are usually referred to as band-pass filters. The filter also often has means for adjusting the coupling factor such, for example, as by a bar made of magnetic material. Because there has been a definite trend in present times to make radio receivers and other electronic apparatus as small as possible in size, there has been an effort made to improve the structure and form of such filters.
To this end, and in order to make such filters small in size, it has been proposed to use a printed circuit technique where the conductors corresponding to the primary and secondary coils are printed on one or both surfaces of a thin insulating plate. Under such circumstances, the insulating plate with the conductors is also used as a capacitor. With the employment of such a printing technique, the filter can be made small in size, but it has been difficult to determine or adjust the mutual inductance or, in other words, the coupling factor. By way of example, in the prior art, the above determination or adjustment is carried out by inserting a magnet into or out of the center of the printed coil or by changing the shape of the coil. With the method of obtaining a predetermined coupling factor by determining the shape of the coil, it is possible to reduce the size of the filter, but this method has been found undesirable because of the difficulty of obtaining a high Q product.
Further, with the prior art filter formed as a printed circuit, and since the electro-magnetic coupling and electro-static coupling are independent of each other, the frequency selection characteristic is not sharp and the spurious signals in the high frequency bands appear with relatively high level.